Formation of carbonate concretions in surface sediments of two mud mounds, offshore Costa Rica - a stable isotope study

Vasileios Mavromatis*, Reiner Botz, Mark Schmidt, Volker Liebetrau, Christian Hensen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The surface sediments of two mud mounds (‘‘Mound 11’’ and ‘‘Mound 12’’) offshore southwest Costa Rica contain abundant authigenic carbonate concretions
dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the
flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18 Ocarbonate values ranging between 34.0 and 37.7 % Vienna standard mean ocean water (VSMOW) and
d13 Ccarbonate values from -52.2 to -14.2 % Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temper-
ature of 4–5 °C. The d18 Ocarbonate values suggest mineral
formation from seawater-derived pore fluid (d18 Oporefluid
= 0 % VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (d18 Oporefluid &5 %) in Mound 11. A positive correlation between d13 Ccarbonate and d18
Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The
carbon of these sources are (1) marine bicarbonate (d13 Cporefluid &0 %) and (2) bicarbonate which formed during the AOM (d13 Cporefluid &-70 %). Furthermore, the d18 Oporefluid composition, with values up to ?4.7 % Vienna
standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boron-enriched fluids. Earlier, it has been shown that the origin of 18 O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (d13
CCH4 = -38 %). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system
interconnected by deep-seated fault(s).
Original languageEnglish
Pages (from-to)1831-1844
JournalInternational Journal of Earth Sciences
Volume103
Issue number7
DOIs
Publication statusPublished - 2014

Fields of Expertise

  • Sonstiges

Treatment code (Nähere Zuordnung)

  • Application
  • Basic - Fundamental (Grundlagenforschung)

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